The strength of bone is a product of the quantity and quality of the tissue. We propose that the compromise in bone strength that cannot be fully explained by a decrease in bone quantity is propagated by inherent defects in the material, resulting in an increased susceptibility to fracture. Similarly, antiresorptive (e.g., bisphosphonates) and anabolic (e.g., PTH) treatments for musculoskeletal diseases may influence both the quantity and quality of the bone matrix, and thus can ultimately improve (or compromise) bone's ability to resist load, but the manner in which this is achieved remains unclear. The underlying hypothesis of this proposal is that subtle modulation of bone's matrix properties, as manifested in chemical composition (e.g., mineral/matrix ratio, calcium/phosphorus ratio, collagen structure, crystallinity) and/or structure will markedly influence the quality of bone, and will result in direct effects on bone structural behavior under mechanical load (e.g., bone stiffness, strength, resilience, toughness). Using a unique combination of state of the art chemical, mechanical, morphological, and histological assays, the primary aim of this study is to identify the principal matrix and architectural factors that define bone quality. These relations will be derived from the rat skeleton defined through aging as well as in situations when the remodeling balance is altered (withdrawal of estrogen) and treated with anti-catabolic or anabolic treatments. These conditions will establish a large range of microscopic and macroscopic tissue properties which will be quantified by in situ synchrotron infrared microspectroscopy and small-angle x-ray scattering to determine chemical properties, synchrotron nano-CT and micro-CT to determine the structure, and nano-indentation and macroscopic mechanical testing regimes to determine mechanical properties. Taken together, these systematic studies present a unique opportunity to first identify and then test precise interrelationships between biochemical, mechanical, and structural factors during aging, hormonal imbalances, and anti-catabolic/anabolic treatment at different hierarchical levels. Identification of these potential chemical targets will provide critical information for improved diagnostic, prophylactic, and therapeutic means of addressing bone quality defects induced by aging, disease, and treatment. The strength of bone is a not only related to the quantity of the tissue but also to its quality. In this project, we will determine the specific material components that determine the mechanic quality of bone. Identification of these potential chemical targets will provide critical information for improved diagnostic, prophylactic, and therapeutic means of addressing bone quality defects in disease.